I don't use it, but I suspect en-5 was originally supposed to be used to denote those who write English professionally, such as journalists, writers, and editors. My own profession would probably qualify. However, I only use the native English one to point out that, yes, I know English as a native, but I find it really unnecessary from a Babel point of view to point out that I can write professionally (especially when my quality of edits on Wikipedia is often subpar).
I don't really see a need for en-5, but I don't begrudge those folks who want to use it.